The present invention relates to image processing, and more particularly to color correction for displaying an image.
In digital image processing, an image is typically represented as a number of pixels. Each pixel's color is defined by the color's coordinates in some color space, e.g. sRGB. The display converts the color coordinates to “grey levels” which are then used to define electrical signals (e.g. voltages) that determine luminous states of corresponding areas on the screen of the display. (Sometimes, the color coordinates themselves can be used as grey levels.) The luminance displayed in the screen area is some function (called “gamma function”, “gamma transfer function”, “gamma transfer characteristic”, or “gamma curve”) of the corresponding grey level.
For many displays, the gamma function is non-linear, and can be approximated by a power relationship:L=vγ  (1)
where L is the luminance, v is the grey level, and γ is a constant for the display. In many CRT (Cathode Ray Tube), LCD (Liquid Crystal Display), and some other types of devices, γ is about 2.2.
For many displays, the relationship (1) is only approximate, and can be corrected using look-up tables (LUTs) containing experimental data obtained for a given display and stored in the display.
A typical color display is associated with a set of primary colors, e.g. red, green and blue. The display accepts separate grey levels for each of the primary colors (i.e. for each “channel”). The gamma functions can be different for different channels, and therefore separate LUTs can be provided for each channel.
For example, a color LCD may include a number of red, green and blue subpixels. The subpixels have identical liquid crystal cells, but have color filters of different colors (red, green, blue). The liquid crystal cells however have different optical activity with respect to wavelength and hence to color. Depending on the spectral bandwidth of the color filters, such optical activity can result in unequal luminance gamma transfer characteristics amongst the R, G, B channels. In addition, the optical activity can result in chrominance deviations (e.g. hue deviations) within each channel. The unequal gamma transfer characteristics can be corrected using the separate LUTs for each channel. The chrominance deviations have not been considered a serious problem for the RGB LCDs due to the narrow spectral bandwidth of the color filters.